A　novel　colorimetric　biosensor,　which　employs　DNAzyme-functionalized　magnetic　beads　(MBs)　as　recognition　probe,　enzyme-assisted　catalytic　oxidation　of　TMB　(3,3′,5,5’-tetramethylbenzidine　sulfate)　as　signal　and　DNA　hybridization　chain　reaction　as　amplification　strategy,　has　been　developed　for　detecting　trace　uranyl　ion　(UO2　2+)　in　seafood　and　aqueous　environment　with　high　sensitivity　and　specificity.　We　demonstrated　that　UO2　2+　can　specifically　cleave　DNAzyme　immobilized　on　MBs　surface　to　release　a　short　single-strand　DNA　(primer),　and　the　released　primer　trigger　DNA　hybridization　chain　reaction　to　form　a　long　one　dimensional　DNA　concatamer　on　the　MBs　surface.　The　resulting　long　DNA　concatamer　could　capture　a　large　amount　of　HRP　to　generate　the　one　UO2　2+-to-multiple　HRP　amplification　effect.　Upon　the　addition　of　TMB-H2O2　solution,　the　HRP-tagged　DNA　concatamer-MBs　conjugates　could　catalyze　the　H2O2-mediated　oxidation　of　TMB,　and　thus　results　in　a　color　change　from　colorless　to　blue　in　solution.　This　provided　a　sensitive　and　selective　sensing　platform　for　the　visual　or　colorimetric　detection　of　UO2　2+.　The　proposed　biosensor　has　high　sensitivity　and　strong　anti-interference　capability,　it　can　be　used　to　detect　as　low　as　2.5 ppb　(9.25 nM)　of　UO2　2+　by　naked-eye　observation　and　0.09 ppb　(0.33 nM)　of　UO2　2+　by　UV-visible　spectrometry　with　no　interference　of　other　ions　and　a　RSD ≤ 6%　(n = 5).　With　the　help　of　this　method,　we　have　successfully　determined　trace　UO2　2+　in　fish　muscle　and　river　water　with　a　recovery　of　93–106%.　High　sensitivity　and　specificity,　as　well　operation　convenience,　low　cost　and　strong　resistibility　to　the　matrix,　which　makes　our　method　a　potential　approach　for　the　on-site　detection　of　UO2　2+　in　seafood　and　aqueous　environment.　©　2016　Elsevier　B.V.